My first nemesis

Good evening.
The achievement Glory of the Delver require de completion of both My first nemesis and My new nemesis.
I completed the second one, when I went to kill zekvir for the first one he didn’t give me credit.
Is it tied to difficulty or other requirements?
Thanks for any information.

My First Nemesis

Defeat Zekvir in his lair before the release of the next season of delves.

This achievement is no longer obtainable. It was during S1.